Proton Temperature Anisotropies in the Venus Plasma Environment During Solar Minimum and Maximum

The proton population in Venus' plasma environment is characterized during periods of solar minimum and maximum using data from a particle mass-energy spectrometer. Such characterizations at different levels of solar activity provides physical insight into solar-cycle-dependent plasma phenomena around the planet, for example mirror modes in the magnetosheath. Statistical distributions of proton bulk speeds and temperatures are generated using a previously developed method which applies Maxwellian fits to measurements of the protons' velocity distribution function. Spatial maps and probability-density histograms comparing the proton parameters between the two time periods are presented. The temperatures perpendicular (T-perpendicular to) and parallel (T-parallel to) to the background magnetic field are found to be 20%-35% lower during solar maximum. Though the overall distributions of the temperature ratio T-perpendicular to/T-parallel to do not change, the regions with higher anisotropy (T-perpendicular to/T-parallel to > 1) are found farther downstream from the bow shock during solar maximum than minimum. This is consistent with the previously observed growth of mirror modes during solar maximum and their decay during minimum.